You could do a script that uses notify-send to popup every x
secs/mins to show you users logged in. e.g.
while [ 1 ]; do
notify-send "Users Logged In" "`who -u`"
Or do your shutdown via commandline. A reminder will automatically popup (until shutdown starts) warning users the system is going down, giving them time to complete/save their work.
shutdown -h +(put number here for how long you want the system to wait before shutdown starts)